Changes in zinc in milk during enzymatic digestion.

Zinc is known to be an essential element for human nutrition and good health1). Milk is its sole source in infants, and growing children as well as mature adults consume varying amounts of milk, which provide them with much of the needed zinc. Investigators, using gel filtration chromatography2,3), have demonstrated that zinc in cow's milk can be associated with fractions of high molecular weight, whereas zinc in human milk is bound to fractions of low molecular weight. A low molecular weight zinc-binding ligand has also been reported in human milk, which, however, is found only in very small amounts in cow's milk. This ligand may improve zinc absorption by normal neonates as well as by children with acrodermatitis enteropathica, a zinc metabolism disorder which is genetic. Hurley et al.4) have suggested that casein is the high molecular weight compound in cow's milk to which zinc mainly binds, and that such bound zinc is less available for absorption. However, no attempt has been made to determine the form in which the zinc in milk can exist in the alimentary tract or the changes which occur on passage of the digesta along the tract. The chemical form of zinc during digestion affects its availability. In the present paper, the authors report the in vitro effects of digestive enzymes and pH on the forms of zinc in milk.